The present invention relates generally to dies for extruding honeycomb bodies and particularly to a method for forming filters, particularly solid particulate filters, such as a diesel exhaust filter.
Honeycomb structures are widely used as in a variety of applications relating to the purification of exhaust gases, including as substrates for catalytic converters and diesel particulate filters. A typical honeycomb structure has a columnar body, the cross-sectional shape of which is typically round or oval. An array of parallel, straight channels is formed by intersecting, interior walls. The channels run axially along the length of the columnar body. The cross-section of each channel can be of any arbitrary shape, but is frequently square.
Extrusion has long been the process of choice for forming honeycomb structures. Conventional dies comprise a feed or inlet section, provided with a plurality of feedholes for the input of a plastic batch material to the die, and a discharge section connecting the feed section for reforming and discharging the material from a discharge face of the die. The discharge openings on the discharge face are formed by criss-crossing array of long straight discharge slots of equal spacing. These long slots intersect to form a network of shorter slot segments which are used to form the cell walls.
Plastic material processed through these dies follow a complex flow path. The material first moves from each feedhole through a transition zone into the base of the slot array, where it flows laterally to join with knit material from adjacent feedholes. Thereafter, the knitted material is again directed forwardly in the direction of feedhole flow toward the discharge opening formed by the slots, being discharge therefrom in the form of an array of interconnecting “webs” or wall portions forming the channels walls of the honeycomb. The cross-sectional shapes of the pins formed on the discharge face by the interconnecting slot segments govern the internal shapes of the channels in the extrudate, and are typically square.
Honeycomb structures have cellular densities which are tailored to specific applications. For diesel particulate filters, cellular densities range between 100 and 300 cells/in2 (15.5 and 46.5 cells/cm2), and more typically 200 cells/in2 (31 cells/cm2) for diesel exhaust filters. Cell wall thicknesses vary between 0.001 to 0.025 in (0.25 to 0.64 mm), and more typically 0.019 in (0.486 mm).
In diesel exhaust filtration, honeycomb structures are plugged at both end faces in a checkerboard pattern. Specifically, alternating cells on one end face of the honeycomb are plugged with a ceramic filter material, and pattern is reverse on the opposite side, so that the ends of each cell are blocked at only one end of the structure. Consequently, when diesel exhaust enters the filter through one end face (i.e., inlet end), it is forced to pass through the porous walls in order to exit through the opposite end face (i.e., outlet end). Hence, diesel particulate filters of this kind are known as wall-flow filters. U.S. Pat. No. 4,420,316 to Frost el al. discusses cordierite wall-flow filters in more detail.
In the manufacturing of wall-flow filters, the honeycomb structures are first extruded as discussed above, and then in a separate step from the extrusion operation, the honeycombs are manually plugged. Specifically, a mask is fitted over one end of the honeycomb and then a paste is spread over the covered end face to fill in the exposed cells. The same process is repeated on the opposite side. This plugging process is labor-intensive and inefficient. Masks are applied manually to the honeycomb ends, and then must be cleaned after each use. The cement batch is time, shear and temperature dependent, often being thrown out, unused due to age restrictions.
From the foregoing, there exists a need for a more efficient method for forming diesel particulate filters. In particular, it is desired to combine the steps of extrusion and plugging in a single operation, and to obtain a die assembly for achieving the same.